1. Field of the Invention
Embodiments of the invention are directed to a method for detecting pulsed laser radiation and to an imaging laser alerter which implements the method. The method and laser alerter are used for the detection and two-dimensional direction determination of incident, pulsed laser radiation, in particular low intensity pulsed laser radiation, which can be caused, e.g., by steering lasers in laser-controlled guided missiles (beam rider guided missiles).
2. Discussion of Background Information
Beam rider guided missiles represent a high-level military threat. They may be detected early in principle on the basis of the steering laser. However, this is made very difficult because of the very low level of laser pulse energy incident at the target, e.g., on the order of magnitude of several fJ/cm2, which is frequently completely covered by the ambient brightness at least during the day. In addition to the early detection, the precise determination, as much as possible to approximately 1°, of the direction of incidence in azimuth and elevation from which the threat comes is also important for possible countermeasures.
According to the general prior art, large-area photodiodes are used as a detector for beam rider detection. Typically, a high limiting frequency of the detection bandwidth is sought by way of the construction of the photodiodes and by special electronic circuitry. To cover the required overall detection angle, typically a plurality of detection sectors—typically quadrants—are each monitored using one detector.
Due to the large area, the total signal intensity incident on the detector is increased in this case. Due to the high limiting frequency, the best possible separation of the pulse signal of the steering laser from the signal of the background brightness is sought. However, since capacitance increases with the area of the photodiodes and limiting frequency of the detection decreases as a result thereof, the achievable signal-to-background ratio of such a detector is limited in principle.
A further substantial disadvantage of this solution is that precise directional detection is not possible, since the number of detection sectors is kept small to limit the technical expenditure. A further disadvantage is that the suppression of the background signal is only possible to a limited extent, since the background light is accumulated over the entire detection sector of a detector in principle. If the sun is located within this sector, beam rider detection is normally hardly possible here, or is only possible with greatly reduced sensitivity.
In a solution approach known from U.S. Pat. No. 7,683,310 A, it is proposed that the directional precision can be increased by using an array of detectors and simultaneously, by using a plurality of detector layers having wavelength separation. In this manner, additional information can be obtained and additional suppression of the background light can be achieved. The disadvantage here is the immensely growing expenditure for a high angular resolution: thus, for an angle determination to 1° for a half space, an array of approximately 27,000 of these detector elements would be necessary, each in turn having a plurality of photodiode layers and corresponding amplifier electronics.
For improved directional determination, in U.S. Pat. No. 5,280,167 discloses the function of the directional determination and the detection are separated. For the detection, a large-area photodiode that detects over one quadrant in each case is again used. Using the pulses received from this detector, a prediction of the next incoming pulse is then generated in a complex logic circuit (the “code breaker”), which then triggers an image amplifier camera. The laser threat can then be localized with higher location resolution and—in the event of correct pulse prediction—lower background signal in the image of this image amplifier camera. A disadvantage of this assembly is that the detection capability of the photodiode detector is limited in principle, as already described above for the solution of a large-area detector element. A further disadvantage is the high expenditure: In addition to the complex code breaker, an image amplifier camera provided with a closure device (“gate”) is required, which additionally only has a small angle of vision to achieve higher location resolutions. As a remedy, it is proposed in U.S. Pat. No. 5,280,167 A that this image amplifier camera be placed on a pivot-tilt head. The expenditure is in turn increased and a time delay arises due to the time required for aligning the pivot-tilt head. The concept is additionally restricted, due to the required pulse prediction, to the detection of accordingly predictable laser threats, which are sufficiently known in their characteristics for this purpose.